Recently, researches regarding the chemical analysis systems, called μ-TAS (micro TAS) or “Lab on a Chip” have been being performed in the fields of chemistry, optics, biotechnology, clinical engineering and the like. In these chemical analysis systems, flow channels are formed on a substrate by fine grooves or holes of a chip (microchip), and series of processing steps, such as mixing, chemical reactions, separation, and detection are all performed on the chip. There are various expectations in these miniaturized chemical analysis systems, such as: reduction in the amounts of samples and waste liquids; reduction of analysis time; improved efficiency; reduced amounts of required space; and portability.
It is necessary to transport fluid into fine flow channels within the miniaturized chemical analysis systems. Fluid transport devices that utilize external forces provided by pumps and the like are commonly employed. In these devices, the influence of the interface tension and the like becomes greater as the flow channels become finer, resulting in increased resistance of the fluid that flow within the flow channels. Therefore, high pressure becomes necessary to cause the fluid to flow. However, there is a possibility that the high pressure result in damage to the fine flow channels, and accordingly it has been necessary to form the flow channels to have a strong structure. In addition, there is a problem that greater power consumption is inevitable in order to generate the high pressure.
For these reasons, a fluid transport device that employs a phenomenon called electrowetting (hereinafter, referred to as “EW”) has been proposed (Patent Document 1). As illustrated in the left half of FIG. 12, when an aqueous electrolyte solution F is in contact with an electrode E, suppose that voltage is applied between the liquid and the electrode. Then, as illustrated in the right half of FIG. 12, by EW, negative ions gather in the proximity of the solid-liquid interface of the liquid in the case that the electrode is an anode, or positive ions gather in the proximity of the solid-liquid interface of the liquid in the case that the electrode is a cathode. The ions repel each other, to reduce the interface tension of the liquid. That is, a contact angle θ between the liquid and the electrode in the left half of FIG. 12 becomes a smaller contact angle θ′ as illustrated in the right half of FIG. 12, due to the reduced interface tension of the liquid. Thereby, the so called wettability of the liquid on the electrode increases.
When the application of voltage is stopped during a state in which the interface tension of the liquid surface is reduced as illustrated in the right half of FIG. 12, the ions which are gathered in the proximity of the interface disperse, the surface tension recovers, and the wettability decreases, as illustrated in the left half of FIG. 12.
Note that the EW phenomenon described above also occurs in cases that an electrode and an aqueous electrolyte solution are not in direct contact, but are insulated by a dielectric film, as illustrated in FIG. 13. This is because the dielectric film becomes polarized such that the side thereof toward the electrode becomes a cathode and the side thereof toward the solution becomes an anode in the case that the electrode is an anode when voltage is applied, or such that the side thereof toward the electrode becomes an anode and the side thereof toward the solution becomes a cathode in the case that the electrode is a cathode when voltage is applied, as illustrated in FIG. 13. Accordingly, the same EW phenomenon as that occurs at the interface between the electrode and the aqueous electrolyte solution occurs at the interface between the dielectric film and the aqueous electrolyte solution.
The fluid transport device leads fluid into a flow channel having hydrophobic inner surfaces, in which electrodes are embedded, by applying voltages to the electrodes and causing the aforementioned EW phenomenon to occur.
Meanwhile, an inkjet printhead that utilizes the EW phenomenon is disclosed (Patent Document 2). In this inkjet printhead, the inner surface of a nozzle near its outlet is covered by a hydrophobic coating. An electric field that progresses toward the outlet along the nozzle is formed to change the surface tension of ink therein, resulting in detaching a drop of ink having a predetermined volume from a continuum of ink supplied. The detached drop of ink is discharged and accelerated electrostatically and therefore ejected from the nozzle.
Patent Document 1:
Japanese Unexamined Patent Publication No. 2005-199231
Patent Document 2:
Japanese Unexamined Patent Publication No. 2004-216899
In the fluid transport device described in Patent Document 1, it is possible to cease movement of the fluid within the flow channel by ceasing the application of voltage. However, the flow channel is filled with the fluid from a reservoir connected to the channel inlet. For this reason, in the case that the aforementioned fluid transport device is utilized in an inkjet printhead or the like, and a material having permeability or hydrophilic properties, such as printing paper, is placed in contact with the channel outlet, the fluid, that is, ink, will be transferred to the material until the material cannot absorb any more ink, or until the reservoir becomes empty. This state is equivalent to that of a fountain pen.
In order for the transfer of the ink onto the material to be ceased in such a case, the channel outlet can be separated from the material, in the same manner that a fountain pen is separated from a sheet. However, if this operation is performed during drawing of intricate patterns, the quality of the patterns may deteriorate, due to slight drag of the contact along the movement direction of the printing paper.
Meanwhile, the printhead disclosed in Patent Document 2 detaches a drop of ink merely by changing the surface tension thereof. Therefore, it is difficult to completely detach the drop within the flow channel.
The present invention has been developed in view of the foregoing circumstances. It is an object of the present invention to provide a fluid transport device, which is capable of completely detaching and measuring off a predetermined amount of fluids within a flow channel.